A Comparative Study On The Ultrastructure Of The Digestive Tract In Different Developmental Periods Of Yellow Mealworm

Tenebrio molitor Linnaeus commonly known as breadworm,it's an important model organism in the order Coleoptera.It is rich in protein and other nutrients,it can be used as feed for animal breeding and has high economic value.In the course of metamorphosis,the larva and the adult have different feeding habits,which are closely related to the structure of digestive tract.Therefore,to study the digestive tract of Tenebrio molitor Linnaeus at different developmental stages,it is expected to provide a reference for the studies on the morphology and development of the digestive tract of Coleoptera insects,and provide a new theoretical basis and reference basis for the development of the digestive tract of Coleoptera insects in different insect stages.In our study,the morphological structure and ultrastructure of alimentary canal of the three life cycles of the mature larvae,pupa and adult of Tenebrio molitor Linnaeus were observed using scanning electron microscope(SEM)and transmission electron microscope(TEM).We studied the characteristics of the components of alimentary canal and the relationship between the components.And we summarized the significant changes of alimentary canal from the larval stage to the pupal stage and finally to the adult stage.In the end,we discussed the adaptability of the structure and function of the alimentary canal of Tenebrio molitor Linnaeus.The main results of our study were as following:1.We used SEM to compare the morphological changes of digestive tract in different developmental stages of Tenebrio molitor Linnaeus.The digestive tract of Tenebrio molitor Linnaeus is a tube that runs through the body cavity of the insect from the mouth to the anus.It consists mainly of the foregut,midgut and hindgut.2.The foregut of Tenebrio molitor Linnaeus is mainly divided into throat,esophagus,crop and proventriculus.The proventriculus is at the last end of the foregut and is the most specialized part of the insect digestive tract.(1)The mature larvae have a well-developed muscle layer with a large number of tracheoles distributed on the outside,and the proventriculus inner wall has eight prominent proventricular fold toward the central axis.(2)When Tenebrio molitor Linnaeus develops from larva to pupa,its eight anterior gastric flaps are significantly thickened and bulged,and these flaps are connected to each other to form a proximal spheroidal proventriculus flap structure.The yellow stomach thorns are sparsely arranged on the proventriculus flap,and the hairy structure is closely arranged at the proximal end of the proventriculus.(3)When the pupae stage develops into the adult stage,the inner wall of the proventriculus flap is thicker,with more dense stomach thorns and hairy structures,and its structure is clearer.Tenebrio molitor Linnaeus is more effective in stirring and breaking down food due to significant changes in the structure of the proventriculus,and it is also convenient to control food into the midgut.3.The midgut of Tenebrio molitor Linnaeus is thicker and larger than the foregut and hindgut.The midgut muscle layer is composed of circular muscle and longitudinal muscle,the muscle layer is underdeveloped and thinner than the foregut and hindgut.(1)There is a small amount of tracheole attached to the muscle layer of the intestinal wall of the mature larvae,and there is no crypt.(2)A small number of smaller regenerative crypts are found on the intestinal wall of the midgut during the A small number of smaller regenerative crypts are found on the intestinal wall of the midgut during the flood season.(3)In the adult stage,the intestinal wall of the midgut is covered with a large number of finger-like crypts,and the arrangement between the crypts is more dense and orderly.The main function of the crypt is to increase the secretion and absorption area of the midgut.At the junction of the midgut and the hindgut,there are six elongated brown-yellow malpighian tubules.4.The muscle layer of the hindgut wall of Tenebrio molitor Linnaeus is developed,the muscle layer is distributed in the same way as the foregut.The circular muscle is outside,and the longitudinal muscle is included.The hindgut includes the ileum,colon,and rectum.The hindgut intestinal wall consists of an epithelial layer and a muscular layer.Its ileum and colonic inner wall are simple,but the epithelial layer of the rectal inner wall is specialized as an inner convex structure.(1)The inner wall of the rectum of the mature larva has a regular and well-arranged cystic inner convex structure.The distal end of the rectum is a contracted rectal tube.The inner wall of the rectal tube forms a triangular conical protrusion in the intestinal lumen,and there is a fine and deep invagination between the protrusions.(2)In the pupal stage,the internal bulge of the inner wall of the rectum is denser than the larval stage.(3)In the adult stage,the inner wall of the rectal epithelial layer is more specialized than the first two stages,and the saccular inner bulge develops into a convex and concave scaly inner bulge.Each scale is arranged closely to form the inner wall of the rectum of the adult.The end of the rectal tube is located at the anus.5.We used TEM to compare and observe the ultrastructure of alimentary canal in different developmental stages of Tenebrio molitor Linnaeus.(1)The muscular layer of the foregut of Tenebrio molitor Linnaeus is well developed,and the epithelial cells of the intestinal wall are flattened.Most interspersed nucleuses are regular ovoid,with a nucleolus in the center.The cells free to the lumen process to form microvilli,there are more microvillus in the foregut of adult than larvae and pupa.(2)The structure of midgut intestinal wall of Tenebrio molitor Linnaeus is simple,muscular layer is not developed,and it lacked of cuticle.Midgut epithelial cell cytoplasm and cell membrane internal folded in stem base,the epithelial cells have abundant microvilli at the end,this may indicated that the cells here are more absorbent than other parts in the digestive tract.The midgut includes the anterior and posterior,its anterior epithelial cells have a large number of mitochondria,rough endoplasmic reticulum and ribosomes.The micro-villi density in the posterior is greater than anterior.Small amounts of lysosomes and vacuoles of different sizes or low electron cloud density vesicles are also distributed in the midgut epithelial cells.(3)The muscle layer of the hindgut of Tenebrio molitor Linnaeus is well developed and has a distinct stratum corneum and lacks microvilli.Endomembrane fold of posterior intestinal epithelium,the rough endoplasmic reticulum was distributed near the inner fold of the posterior intestinal epithelial membrane,which may be related to the synthesis of lipids,proteins and polysaccharides.There are a large number of low electron cloud density vesicles or vacuoles and a small number of organelles in the hindgut epithelial cells.Combined with the above characteristics of the hindgut of Tenebrio molitor Linnaeus,it is further explained that the hindgut is the main site for reabsorption of substances such as water and inorganic salts.